There are many situations in which it is desirable to sense the electrostatic potential of an object relative to a reference electrostatic potential. The present invention relates to an electrostatic sensor which can be used as a contact or non-contact type sensor. The term "non-contact" refers to the type of electrostatic potential sensor in which the sensing electrode is not brought contact with the object being sensed but only next to it so that a capacitance exists between the sensed object and the electrode. Shu, C. H. and Muller, R. S., "Micromechanical Electrostatic Voltmeter," 7th International Conference on Solid-State Sensors and Actuators (Transducers), San Fransisco, June 1991, pp. 659-662, disclose a micromachined non-contacting electrostatic voltmeter which works on the principal of intermittent shuttering and exposing a sensing electrode to an electric field between a remote electrode at a different potential and the sensing electrode. In particular, a sensing electrode is formed on the surface of a substrate. A polysilicon microshutter is cantilevered above the detector electrode on anchors. The shutter is rigidly coupled to a series of fingers which are suspended on the opposite side of the anchors. Those fingers are interdigitated with stationary fingers. The stationary fingers are coupled to an AC driving voltage which causes the entire suspended microstructure to oscillate. The oscillation of the suspended structure (which includes the fingers and the shutter) causes the shutter to continuously change, in a sinusoidal pattern, the area of the detector electrode which is overhung by the shutter. Suspended above the sensing electrode and the shutter is an input electrode which is electrically coupled via a conductor to the remote object to be sensed. The sensing electrode is coupled to an amplifier connected as a current-to-voltage converter. The output of the amplifier is forwarded to signal processing circuitry for measuring electrostatic potential. The oscillation of the shutter plate causes the voltage at the detector electrode to vary sinusoidally. The amplitude of the oscillation is a function of the voltage difference between the voltage at the input electrode (the voltage of the object being sensed) and ground.
Loconto, D. P. and Muller, R. S., "High-Sensitivity Micromechanical Electrostatic Volt Meter," 7th International Conference on Solid State Sensors and Actuators, 1993, pp. 878-881, disclose a second generation version of the device disclosed in the above-identified paper.
The voltmeters disclosed in these two references are not particularly sensitive. Further, it is more preferable to differentially sense electrostatic potential as it allows for greater resolution by rejecting common mode noise in the system. Even further, these sensors are complicated to fabricate because they require two separate suspended layers, with one above the other.